THe 4wd car is going to drain a battery a lot faster than running a 2wd... I run a 6000mah / 120c in my Tekno baby truggy and with a 10 min main I usually will have 25-30% left in the battery.

As far as your 'limp' mode... I am guessing your esc is hitting low voltage cut-off... you may have it set to high at 3.6v per cell as you stated the battery was ready 7.2v. Check you esc for what it is set at.